Apparatus for concentrating sludge

ABSTRACT

A sludge concentrating apparatus comprising a sludge tank for storing sludge and a filter cloth having a shape of endless belt which turns between upper and lower rollers, further comprising filtering means of which at least the lowermost of rollers is immersed in the sludge, washing means of the filter cloth, and a filtrate chamber having a filtrate introducing port in contact with the filter cloth and a weir, wherein a sludge distributing tank serving as sludge introducing means into the sludge tank is provided near the upper end of the filtrate introducing port, and a concetrated sludge discharge tank serving as concertrated sludge introducing means with the filter cloth in between; said sludge concentrating apparatus further comprising coagulant adding means for adding an acid, and a flow adjustment device serving as a sludge uniform feeding means which feeds sludge fed into the sludge tank uniformly in the width direction of the filter cloth. It is thus possible to efficiently concentrate sludge produced in a sewage disposal plant in a large quantity without depending upon properties of sludge.

This application is a U.S. National Phase Application under 35 U.S.C.371 of International Application PCT/JP02/04654, filed May 14, 2002.

TECHNICAL FIELD

The present invention relates to a method for concentrating sludge andan apparatus thereto. More particularly, the invention relates to amethod for concentrating sludge and an apparatus used in said method forconcentrating sludge which permit subjecting sludge occurring, forexample, in a sewage disposal plant to solid/liquid separation and aconcentrating treatment in a large quantity and at a high efficiency,without depending upon properties of sludge.

BACKGROUND ART

A sludge concentrating method commonly applied in a general sewagedisposal plant will now be described with reference to the drawings.

FIG. 16 is a flowchart illustrating the sludge concentrating method inthe sewage disposal plant.

As shown in FIG. 16, sewage flowing into a primary settling tank 1 issubjected to a solid/liquid separation through a gravity precipitatingtreatment. Then, supernatant water after the precipitation treatment isfed to a reaction tank 2, where it is subjected to an activated sludgeconcentration. The sewage thus treated in the activated sludgeconcentration is fed to a secondary settling tank 3, where it issubjected again to a solid/liquid separation through a gravityprecipitating treatment. The resultant supernatant water obtained in thesecondary settling tank 3 is fed to a sterilizing tank 4, where it issterilized. The treated water resulting from sterilization of sewage isdischarged to rivers, lakes or sea water.

On the other hand, sedimentation sludge resulting from the primarysettling tank 1 and excess sludge resulting from the secondary settlingtank 3 are usually mixed. The resultant mixed sludge is fed to a sludgeconcentrator 5. The mixed sludge fed to the sludge concentrator 5 hasusually a solid concentration of about 1% or so. The mixed sludge issubjected to a concentration treatment in the sludge concentrator 5through gravity precipitation. This results in a concentrated sludgehaving a solid concentration of about 2 to 3%. The resultantconcentrated sludge is stored in a sludge storage tank 6. The sludgestored in the sludge storage tank 6 is dehydrated by a dehydrator 7.Dehydrated cakes are thus obtained.

In an ordinary sewage disposal plant, as described above, gravityprecipitation is used for concentration treatment of mixed liquid ofsewage flowing down and sludge (hereinafter referred to as “sludge”).More specifically, the sewage flowing down is subjected to solid/liquidseparation by gravity precipitation in the primary settling tank 1. Thesewage concentrated through activated sludge is subjected to thesolid/liquid separation by gravity precipitation in the secondarysettling tank 3. The mixed sludge is concentrated by gravityprecipitation in the sludge concentrator 5.

The above-mentioned sludge concentrating method based on gravityprecipitation requires only small power consumption, thus providing anadvantage of a low treatment cost. However, the apparatus for sludgeconcentration based on gravity precipitation has a large area ofinstallation, and has a problem in that the treatment ability of theapparatus largely depends upon properties of sludge and the watertemperature to be treated.

In recent years, particularly, precipitation property of sludge isbecoming worse by causes such as increase of organic matters in sludgeand putrefaction of sludge transported over a long distance as a resultof achievement of intensive disposal of sludge. Consequently, it is thecurrent reality that in the sludge concentrator in a sewage disposalplant conducting intensive disposal of sludge, it is becoming moredifficult to ensure a concentrated sludge constantly having a solidconcentration of 2% or higher through sludge concentration method basedon gravity precipitation.

When a concentrated sludge of a prescribed concentration is unavailable,the amount of mixed liquid of sludge fed to dehydrating equipment in thelatter step becomes larger. This not only makes it difficult to operatethe dehydrator in stable, but also requires excessive facilities fordehydration apparatus.

For the purpose of solving these problems, mechanical concentratingapparatuses such as a centrifugal separator are being adopted recently,but this method is economically unfavorable because of the large powerconsumption.

In high-level treatment methods such as removal of nitrogen orphosphorus of which the positive adoption is currently considered with aview to preventing eutrophication of public waters, the reaction tank isoperated with a higher solid concentration than in the standardactivated sludge process in many cases in order to increase a reactionrate. It becomes therefore necessary to apply a solid/liquid separationtreatment to sludge-mixed liquid having high concentration flowing fromthe reaction tank. In a metropolis in particular, where many secondarysettling tanks coping with the standard activated sludge process arealready in practical operation, adoption of a high-level treatmentrequires replacement of the secondary settling tank 3 in FIG. 16, ordevelopment of new sludge concentrating means capable of complementingthe same.

Demand for a technique permitting concentration of sludge for certainwithout relying upon gravity precipitation is now increasing asdescribed in the foregoing.

From the above-mentioned point of view, the present inventors developeda sludge concentrating apparatus disclosed in Japanese Unexamined PatentApplication Publications Nos. 11-216312, 2000-5506 and 2000-262817.

The present inventors further studied the above-mentioned conventionalsludge concentrating apparatus, and obtained the following findings. Byadding the coagulant, then slowly transferring the coagulated flock, anduniformly feeding the same in the width direction of filter cloth, it ispossible to take large steps toward improving efficiency ofconcentration of sludge, and they completed the present invention.

That is, the present invention has an object to provide a sludgeconcentrating apparatus which permits, when disposing of sludge by usingthe above-mentioned conventional sludge concentrating apparatus,efficient concentrating treatment in a large quantity without dependingupon properties of sludge.

DISCLOSURE OF INVENTION

A first invention provides a method for concentrating sludge, whichconcentrates a mixed sludge of a sludge in a primary settling tank andan excess sludge by using a sludge concentrating apparatus whichcomprises a sludge tank storing sludge, and a filter cloth having ashape of endless belt turning between a plurality of rollers, furthercomprises a filtrate chamber having filtering means in which at leastthe lowermost of the rollers is immersed in the sludge, washing means ofthe filter cloth and means for keeping a filtrate introducing port inclose contact with the filter cloth and the liquid level at a constantstate, wherein sludge introducing means to the sludge tank is providednear the upper end of the filtrate introducing port, and concentratedsludge discharge means from the sludge tank is provided at a positionsubstantially opposite to the sludge introducing means with the filtercloth in between; the sludge concentrating method being characterized inthat an organic macromolecular coagulant is previously added to themixed sludge.

A second invention provides a method for concentrating sludge whichconcentrates an excess sludge by using a sludge concentrating apparatuswhich comprises a sludge tank storing sludge, and a filter cloth havinga shape of endless belt turning between a plurality of rollers, furthercomprises a filtrate chamber having filtering means in which at leastthe lowermost of the rollers is immersed in the sludge, washing means ofthe filter cloth and means for keeping a filtrate introducing port inclose contact with the filter cloth and the liquid level at a constantstate, wherein sludge introducing means to the sludge tank is providednear the upper end of the filtrate introducing port, and concentratedsludge discharge means from the sludge tank is provided at a positionsubstantially opposite to the sludge introducing means with the filtercloth in between; the sludge concentrating method being characterized inthat an inorganic coagulant and an organic macromolecular coagulant arepreviously added to the excess sludge.

A third invention provides the sludge concentrating method according tothe second invention, wherein poly ferric sulfate is used as theinorganic coagulant.

A fourth invention provides the sludge concentrating method according toany one of the first to third inventions, wherein a cationic coagulantis used as the organic macromolecular coagulant.

A fifth invention provides a sludge concentrating method comprising astep of adding an acid and an inorganic coagulant to sludge, and a stepof concentrating the sludge to which the acid and the inorganiccoagulant has been added.

A sixth invention provides the sludge concentrating method according tothe fifth invention, wherein the acid is sulfuric acid and the inorganiccoagulant is poly ferric sulfate.

A seventh invention provides the sludge concentrating method accordingto the fifth or sixth invention, wherein supernatant water, separatedwater or filtrate has a pH within a range from 4 to 6.

An eighth invention provides a sludge concentrating apparatus whichcomprises a sludge tank storing sludge, and a filter cloth having ashape of endless belt turning between a plurality of rollers, furthercomprises a filtrate chamber having filtering means in which at leastthe lowermost of the rollers is immersed in the sludge, cleaning meansof the filter cloth and means for keeping a filtrate introducing port inclose contact with the filter cloth and the liquid level at a constantstate, wherein sludge introducing means to the sludge tank is providednear the upper end of the filtrate introducing port, and concentratedsludge discharge means from the sludge tank is provided at a positionsubstantially opposite to the sludge introducing means with the filtercloth in between.

A ninth invention provides the sludge concentrating apparatus accordingto the eighth invention, further comprising coagulant adding means whichadds the coagulant to the sludge fed into the sludge tank, and acidadding means which adds the acid.

A tenth invention provides the sludge concentrating apparatus accordingto the ninth invention, wherein the acid is sulfuric acid, and theinorganic coagulant is poly ferric sulfate.

An eleventh invention provides the sludge concentrating apparatusaccording to any one of the eighth to tenth inventions, furthercomprising sludge uniform feeding means which supplies sludge fed intothe sludge tank uniformly in the width direction of the filter cloth.

A twelfth invention provides the sludge concentrating apparatusaccording to the eleventh invention, wherein the sludge uniform feedingmeans has at least any one of a submerged weir and a flow adjustmentdevice.

A thirteenth invention provides the sludge concentrating apparatusaccording to any one of the eighth to twelfth inventions, wherein thedistance between the inner wall surface of the sludge tank atdischarging side of the concentrated sludge and the outer surface of thefilter cloth is smaller than the distance between the inner wall surfaceof the sludge tank at the sludge feeding side and the outer surface ofthe filter cloth.

A fourteenth invention provides the sludge concentrating apparatusaccording to any one of the eighth to thirteenth inventions, furthercomprising sludge uniform discharge means which discharges sludge fromthe sludge tank uniformly in the width direction of the sludge tankwithout pilling up the concentrated sludge in the sludge tank and adeposit contained therein.

A fifteenth invention provides the sludge concentrating apparatusaccording to the fourteenth invention, wherein the sludge uniformdischarge means is provided in the downstream of an exit weir of theconcentrated sludge discharge means and has a roller and depositstripping means.

A sixteenth invention provides the sludge concentrating apparatusaccording to any one of the eighth to fifteenth inventions, wherein thecleaning means has a plurality of cleaning spray nozzles arranged in astaggered form alternately with a single or a plurality of header pipes.

A seventeenth invention provides the sludge concentrating apparatusaccording to any one of the eighth to sixteenth inventions, wherein thecleaning means is oscillatory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, as viewed from the filter cloth descendingside, illustrating a sludge concentrating apparatus of the presentinvention;

FIG. 2 is a perspective view, as viewed from the filter cloth ascendingside, illustrating a sludge concentrating apparatus of the presentinvention;

FIG. 3 is a sectional view illustrating a sludge concentrating apparatusof the present invention;

FIG. 4 is a; perspective view, as viewed from the filter clothdescending side, illustrating another sludge concentrating apparatus ofthe present invention, wherein a filtrate chamber is provided in thesludge tank;

FIG. 5 illustrates the method for adding an acid and a coagulant tosludge;

FIG. 6 is a perspective view, as viewed from the filter cloth descendingside, illustrating still another sludge concentrating apparatus of thepresent invention, wherein sludge uniform feeding means is provided;

FIG. 7 is a sectional view illustrating still another sludgeconcentrating apparatus of the present invention, wherein sludge uniformfeeding means is provided;

FIG. 8 is a perspective view, as viewed from the filter cloth descendingside, illustrating another sludge concentrating apparatus of the presentinvention, wherein a filtrate chamber is provided in the sludge tank;

FIG. 9 is a perspective view, as viewed from the filter cloth ascendingside, illustrating another sludge concentrating apparatus of the presentinvention, wherein a sludge uniform discharge means is provided;

FIG. 10 is a sectional view illustrating another sludge concentratingapparatus of the present invention, wherein sludge uniform dischargemeans is provided;

FIG. 11 is a sectional view illustrating a sludge concentratingapparatus of the present invention having cleaning spray nozzlesarranged in a staggered form;

FIG. 12 is a plan view illustrating cleaning means having cleaning spraynozzles arranged in a staggered form;

FIG. 13 is a perspective view illustrating cleaning means havingcleaning spray nozzles arranged in a staggered form;

FIG. 14 is a plan view illustrating cleaning means having oscillatorycleaning spray nozzles;

FIG. 15 is a perspective view illustrating cleaning means havingoscillatory cleaning spray nozzles;

FIG. 16 is a flowchart illustrating a sludge concentrating methodapplied in a sewage disposal plant.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the sludge concentrating apparatus for theimplementation of the sludge concentrating method of the presentinvention will be described with reference to the drawings.

FIG. 1 is a perspective view, as viewed from the filter cloth descendingside, illustrating a sludge concentrating apparatus of the presentinvention; FIG. 2 is a perspective view, as viewed from the filter clothascending side, illustrating the sludge concentrating apparatus of thepresent invention; FIG. 3 is a sectional view illustrating the sludgeconcentrating apparatus of the present invention; FIG. 4 is aperspective view, as viewed from the filter cloth descending side,illustrating another sludge concentrating apparatus of the presentinvention wherein a filtrate chamber is provided in the sludge tank; andFIG. 5 illustrates the method for adding an acid and a coagulant tosludge.

In FIGS. 1 to 4, reference numeral 8 represents a sludge tank having asludge feed pipe 9 and a concentrated sludge discharge pipe 10. Sludgeis fed from the sludge feed pipe 9 via an inlet weir described laterinto the sludge tank 8.

Reference numeral 11 represents a filtrate chamber provided in thesludge tank 8. Outside the sludge tank 8, an exit side chamber 11A isformed integrally with the filtrate chamber 11. A filtrate dischargepipe 12 is provided in the exit side chamber 11A. A filtrate introducingport 14 communicating with the sludge tank 8 is formed on each sidewallfacing each other of the filtrate chamber 11. Supply of sludge into thesludge tank 8 and discharge of filtrate therefrom are adjusted so thatthe liquid level of sludge in the sludge tank 8 is always above that inthe filtrate chamber 11. This is to filter sludge in the sludge tank 8through natural flow from the filtrate introducing port 14 via afiltering means described later into the filtrate chamber 11. Pressurein the filtration can be adjusted by adjusting the difference betweenthe sludge level in the sludge tank 8 and the liquid level in thefiltrate chamber 11. A weir 13 for keeping a constant liquid level inthe filtrate chamber 11 is provided at the boundary with the exit sidechamber 11A of the filtrate chamber 11.

For the purpose of further improving the filtering efficiency of sludge,a plurality of filtrate chambers 11 may be formed in the sludge tank 8.When forming a plurality of filtrate chambers 11, at least one filtrateintroducing port 14 is formed in each filtrate chamber 11. There is noparticular limitation on the shape of the filtrate introducing port 14,but it should preferably be shaped into a lattice form in considerationof strength.

As shown in FIG. 4, the filtrate chamber 11 may be provided completelywithin the sludge tank 8. In this case, a riser tube 12A formed byextending the filtrate discharge pipe 12 into the filtrate chamber 11and arranging it vertically, and having a telescopic valve at theleading end thereof can be adopted, as shown in FIG. 4, as means foralways keeping a constant liquid level in the filtrate chamber 11. Byproviding the filtrate chamber 11A completely within the sludge tank 8,it is possible to downsize the sludge concentrating apparatus.

Reference numeral 15 represents filtering means having the lower partimmersed in the sludge tank 8. The filtering means 15 is for filteringsludge fed into the sludge tank, and has a filter cloth 18 forming anendless belt which turns endlessly in the vertical direction under theeffect of driving means 27 between a lower roller 16 and an upper roller17 while covering the filtrate introducing port 14. The filter cloth 18is made of polyester, polypropylene or the like, and turns in adirection and continuously while covering the filtrate introducing port14.

Reference numeral 23 represents a sludge distributing tank having ashape enlarged upward in the tapered form as sludge introducing means.It is provided on a sidewall of the sludge tank 8 on the filter clothdescending side, and has a sludge feeding pipe 9. An inlet side weir 28is provided at a sludge discharge flow port of the sludge distributingtank 23. As a result, sludge fed from the sludge feeding pipe 9 isuniformly supplied in the width direction of the filter cloth 18 in thesludge tank 8.

Reference numeral 24 represents solid stripping means for removing solidmaterials adhering to the filter cloth 18. The solid stripping means 24is installed above the sludge level in the sludge tank 8, and graduallystrips off the solid materials adhering to the ascending filter cloth18. As the solid stripping means 24, stripping means based on scraperhaving a structure of scraping solid sludge cakes adhering to the filtercloth 18 by means of a scraper is the most desirable. Other applicablemeans include stripping means based on air, i.e., stripping means ofremoving sludge cakes adhering to the filter cloth 18 by ejecting airtoward the filter cloth 18, stripping means based on vibration, i.e.,stripping means of stripping sludge cakes adhering to the filter cloth18 by causing the filter cloth 18 to vibrate mechanically or byultrasonic waves. Stripped solid materials fall into the sludge tank 8,and recovered from the concentrated sludge discharge pipe 10 via theabove-mentioned exit side weir.

Reference numeral 19 represents nozzle-type cleaning means for cleaningthe filter cloth 18. The cleaning means 19 is installed above the solidstripping means 24 on the sludge surface on the filter cloth ascendingside in the: sludge tank 8, and further removes solid materials whichcould not be stripped off by the solid stripping means 24. The cleaningmeans 19 should preferably be cleaning means based on water, i.e., thecleaning means of removing sludge cakes adhering to the filter cloth byejecting water toward the filter cloth 18. Discharge water in thecleaning produced by the cleaning means 19 is recovered by cleaningdischarge water recovering means 26.

The cleaning discharge water recovering means 26 is composed of abox-shaped receiving tank having an opening upper ceiling board, andcollects cleaning discharge water by installing the cleaning means 19 inthis receiving tank or on the upper part of the receiving tank. The thuscollected cleaning discharge water is returned to the sludge tank 8 onthe filter cloth descending side for filtration again.

Reference numeral 25 represents a concentrated sludge discharge tankserving as concentrated sludge discharge means, and has a concentratedsludge discharge pipe 10. The concentrated sludge discharge tank 25 hasan exit side weir 31 for recovering concentrated sludge which isconcentrated in the sludge tank 8. By installing the exit side weir 31so as to project toward the sludge tank 8 on the side of the sludge tank8 on the filter cloth ascending side, concentrated sludge can becollected uniformly in the width direction of the filter cloth 18through natural flowing. By causing the exit side weir 31 to projecttoward inside of the sludge tank 8, the solids stripped off by the solidstripping means 24 fall onto the exit side weir 31, and are collectedtogether with concentrated sludge.

Reference numeral 21 represents filter cloth tensioning means comprisinga pair of stretching rollers 20, which give an appropriate tension tothe filter cloth 18. Since the filter cloth 18 may be elongated throughcontinuous running for a long period of time, an appropriate tension isconstantly imparted to the filter cloth 18 by filter cloth tensioningmeans 21. Means using an air cylinder is the most desirable as filtercloth tensioning means 21. This type permits constant tensioning of thefilter cloth 18 with a uniform force, and installation of tensiondetector means enables to take an emergency stop action upon breakage ofthe filter cloth 18.

Provision of meandering prevention means for the filter cloth 18 makesit possible to maintain a stable operation of the filter cloth 18. Themeandering prevention means should preferably be a nip pressure typemeandering correcting apparatus commonly used for automatic coiling offibers or paper. This apparatus corrects the position of the filtercloth 18 always at the center by arranging a pair of nip-pressure typemeandering correctors on each side of the filter cloth 18 so that eachof the right and left nips pulls the filter cloth 18 toward the end, andwhen meandering of the filter cloth 18 occurs by some cause or other andthe cloth leans to the right or the left, reducing the nip pressure onthe leaning side.

Reference numeral 22 represents sealing means for sealing the filtrateintroducing port 14. While the filtrate introducing port 14 is usuallycovered with the filter cloth 18, sludge may mix directly into thefiltrate chamber 11 without passing through the filter cloth at thewidth direction end of the filter cloth 18. This is prevented by use ofthe sealing means 22. When using the sealing means 22, it is desirableto press it against the boundary between the width-direction end of thefilter cloth 18 and the filtrate introducing port 14 so that running ofthe filter cloth is not prevented by the resin plate.

Reference numeral 29 represents organic macromolecular coagulant addingmeans for adding an organic macromolecular coagulant to sludge. Foradding the organic macromolecular coagulant to sludge, it is desirableto provide organic macromolecular coagulant storing means separately,and line-inject the organic macromolecular coagulant to the sludge feedpipe 9.

Reference numeral 30 represents inorganic coagulant adding meansnecessary when adding an inorganic coagulant to sludge. For adding theinorganic coagulant to sludge, it is desirable to provide inorganiccoagulant storing means separately, and line-inject the inorganiccoagulant to the sludge feed pipe 9.

For adding the coagulant, the most desirable method is to first add theinorganic coagulant to the sludge to be disposed of, and then add theorganic macromolecular coagulant. It is also possible to apply a methodof adding simultaneously the inorganic coagulant and the organicmacromolecular coagulant considering the piping of the plant, or amethod of first adding the organic macromolecular coagulant, and thenadding the inorganic coagulant.

The method for concentrating sludge of the present invention by mean ofthe above-mentioned sludge concentrating apparatus will now bedescribed.

When the sludge to be disposed of is exclusively sludge obtained in theinitial sedimentation basin or a mixed sludge of an excess sludge andthe initial sedimentation basin sludge, the coagulant added to thesludge to be disposed of should preferably comprise exclusively anorganic macromolecular coagulant. Preferable organic macromolecularcoagulants added by the organic macromolecular coagulant adding means 29include a cationic coagulant, a nonionic coagulant, and an anioniccoagulant. A cationic coagulant is particularly preferable.

When the sludge to be disposed of exclusively comprises an excesssludge, or when sufficient filtering and concentration cannot beaccomplished with an organic macromolecular coagulant alone, an organicmacromolecular coagulant and an inorganic coagulant are usedsimultaneously. The inorganic coagulant to be added to sludge shouldpreferably be a coagulant mainly comprising iron or aluminum, and polyferric sulfate is particularly preferable because of the possibility touse against bad odor and to use for removal of phosphorous The ratio ofaddition of the inorganic coagulant, varying with properties of sludge,should preferably be within a range from 5 to 30% relative to the solidcontent in sludge, or particularly preferably, within a range from 5 to15.%.

The ratio of addition of the organic macromolecular coagulant, varyingwith properties of sludge, should preferably be within a range from 0.01to 0.2% relative to the solid content in sludge when simultaneouslyusing with an inorganic coagulant, or particularly preferably, within arange from 0.05 to 0.1%. When employing exclusively the organicmacromolecular coagulant, the preferable range is from 0.1 to 0.3%relative to the solid content in sludge, or a particularly preferablerange is from 0.1 to 0.2%.

The sludge to which at least any of the inorganic coagulant and theorganic macromolecular coagulant is fed from the sludge feed pipe 9 intothe sludge distributing tank 23. Since an inlet side weir 28 is providedin the sludge distributing tank 23, sludge is fed into the sludge tank 8uniformly in the width direction of the filter cloth 18.

When mixed sludge flows into the sludge tank 8, solid materials in themixed sludge are filtered by the filter cloth 18 running continuously,and the filtrate naturally flows from the filtrate introducing port 14into the filtrate chamber 11. Because the filter cloth 18 runs whilecovering the filtrate introducing port 14, and in addition, sealingmeans is provided, there is no risk of the mixed sludge in the sludgetank 8 directly flowing into the filtrate chamber 11. As a result, themixed sludge in the sludge tank 8 is gradually concentrated from thefilter cloth descending side toward the filter cloth ascending side.

The thus concentrated sludge passes through the exit side weir 31provided in the sludge tank 8 on the filter cloth ascending size, and isdischarged outside the sludge tank 8 from the concentrated sludgedischarge pipe 10. On the other hand, the filtrate flowing into thefiltrate chamber 11 is discharged, through the weir 13 in the case ofthe sludge concentrating apparatus shown in FIG. 1, and through theriser tube 12A in the case of the sludge concentrating apparatus shownin FIG. 4, from the filtrate discharge pipe 12 to outside the filtratechamber 11, and collected. In all cases, the filtrate level in thefiltrate chamber 11 is kept constant by the weir 13 or the riser tube12A.

Along with the progress of filtration, sludge cakes adhere to the filtercloth 18 and accumulate there. These sludge cakes are gradually strippedoff by the solid stripping means 24, discharged from the exit side weir31 to outside the sludge tank 8, and collected together withconcentrated sludge. While most of the adhering sludge cakes arestripped off by the solid stripping means 24, sludge cakes not strippedoff are gradually removed by the cleaning means 19 and regenerated. Thefiltering efficiency of the filter cloth 18 is therefore never reduced,and the filtering means 15 is constantly operated stably.

As cleaning discharge water produced by the cleaning means 19 iscollected by the cleaning discharge water collecting means 26, theconcentrated sludge is never diluted on the filter cloth ascending sideas a result of mixture of cleaning discharge water into the sludge tank8 on the filter cloth ascending side. Cleaning discharge water collectedby the cleaning discharge water collecting means 26 is returned to thesludge tank on the filter cloth ascending side and filtered again.

When disposing of mixed sludge by adding a coagulant thereto, filtratehas a higher clarity than when disposing thereof without adding acoagulant. When using water for the cleaning means 19, therefore, thefiltrate in the filtrate chamber 11 can be used as cleaning water. Bythus using the filtrate in the filtrate chamber 11, it is possible toprevent the consumption of disposal water from increasing.

Provision of filter cloth tensioning means 21 and filter clothmeandering preventing means eliminates the risk of racing or meanderingof the filter cloth 18 even upon occurrence of elongation of the filtercloth 18. This permits achievement of continuous running of the filtercloth 18, thus ensuring safe operation for a long period of time. Evenupon breakage of the filter cloth, the apparatus can beemergency-stopped, thus providing remarkable merits in practice.

In respect of automatic control of the apparatus, it becomes possible toperform continuous disposal with a constant solid concentration of theconcentrated sludge, thus facilitating the dehydrating operation in thenext dehydration step by providing solid concentration detecting meansand feed amount detecting means of sludge fed to the sludgeconcentrating apparatus, and in addition, by providing means detectingthe solid concentration and the amount of discharge of the filtratedischarged from the filtrate chamber 11.

When concentrating sludge by use of the above-mentioned sludgeconcentrating apparatus, an acid may be added in addition to thecoagulant added to the sludge.

The acid should preferably be sulfuric acid or hydrochloric acid. Whenthe coagulant used is based on sulfuric acid, however, it is desirableto use sulfuric acid, and to use hydrochloric acid in the case of ahydrochloric acid based coagulant. The combination of poly ferricsulfate and sulfuric acid is particularly preferable. The feed amount ofthe acid is adjusted by measuring pH of supernatant water, separatedwater or filtrate from the above-mentioned sludge concentratingapparatus so that pH has a certain value (from 4 to 6). Since thisensures a constant feed amount of the inorganic coagulant, it ispossible to reduce the feed amount of the inorganic coagulant, and toconduct stable solid/liquid separation and concentration.

The method for adding the coagulant and the acid to sludge comprises, asshown in FIG. 5, the steps of providing an inorganic coagulant storagetank 32 and an acid storage tank 33, sending the acid in the acidstorage tank 33 into a pipeline 36 or a pipeline 37 having a feedingpump 38 by means of a feeding pump 35, mixing the sludge and theinorganic coagulant, and feeding the resultant mixture to the sludgeconcentrator. The point where the acid is fed to the pipeline 36 via thepipeline 34 may be before or after the supply of the inorganic coagulantto the pipeline 36 from the pipeline 37. Preferably, it should be beforesupply of the inorganic coagulant, as shown in FIG. 5.

EXAMPLES

The present invention will now further be described by means ofexamples.

Example 1

A mixed sludge comprising a mixture of sludge in a primary settling tankand an excess sludge of a sewage disposal plant (solid content in excesssludge: 50%) was concentrated. The mixed sludge had an SS concentrationof 0.5% (in average). A cationic coagulant was added as an organicmacromolecular coagulant in a fixed amount of 0.1% relative to the solidconcentration. For comparison, concentration was carried out by addingan inorganic coagulant (poly ferric sulfate only) in a fixed amount of15% relative to the solid concentration. The result is shown in Table 1.

TABLE 1 Solids SS Concentration concentration of in Concentratedfiltrate Throughput sludge Inorganic coagulant 90 mg/l 30 m³/h 3.9%(Comparative Example) Organic macromolecular 40 mg/l 65 m³/h 4.1%coagulant (Example of Invention

As is known from Table 1, in the case of the Example of the presentinvention, the SS concentration of the filtrate is lower, with a betterconcentration efficiency, and the throughput is larger as compared withthe Comparative Example.

Example 2

An excess sludge of a sewage disposal plant was concentrated. The excesssludge had an SS concentration of 0.8% (in average). As an inorganiccoagulant, poly ferric sulfate was added in a fixed amount of 15%relative to the solid content. As an organic macromolecular coagulant, acationic coagulant was added in a fixed amount of 0.05% relative to thesolid content. As a comparative example, concentration was conducted byadding only an inorganic coagulant (poly ferric sulfate) in an amount of15% relative to the solid content. The result is shown in Table 2.

TABLE 2 Solids SS Concentration concentration of of Concentratedfiltrate Throughput sludge Inorganic coagulant only 140 mg/l 23 m³/h3.8% (Comparative Example) Inorganic coagulant and  55 mg/l 51 m^(3/)h4.0% organic macromolecular coagulant added (Example of Invention)

As is clear from Table 2, in the case of Example of the presentinvention, the SS concentration of supernatant water is lower, with abetter concentration efficiency, and the throughput is larger than inComparative Example.

Another example of the present invention in which an acid was added tosludge and sludge was concentrated in the above-mentioned sludgeconcentrating apparatus will now be described.

Example 3

A mixed sludge comprising a mixture of sludge in a primary settling tankand an excess sludge of a sewage disposal plant was concentrated by thegravity concentrating method. The mixed sludge had an SS concentrationof 0.9% (in average), and a PO₄—P concentration of 36 mg/l. Sulfuricacid was added as an acid so that supernatant water had a pH of 6. Polyferric sulfate was added as a coagulant in a fixed amount of 15%relative to the solid content. As a Comparative Example, sludge wasconcentrated by adding only a coagulant (poly ferric sulfate) in a fixedamount of 20% relative to the solid content without adding an acid oranother coagulant. The result is shown in Table 3.

TABLE 3 SS PO₄—P Solids concentration concentration Concentration ofsupernatant of supernatant of Concentrated water water sludge Acid orcoagulant 1760 mg/l 35 mg/l 1.8% not added (Comparative Example) Acidnot added and  850 mg/l  3 mg/l 2.1% coagulant added (ComparativeExample) Acid and coagulant  210 mg/l  2 mg/l 2.4% added (Example ofInvention)

As is clear from Table 3, in the Example of the present invention inwhich an acid and a coagulant were added, the SS concentration ofsupernatant water was lower, with a better concentration efficiency, andPO₄—P could sufficiently be removed as compared with the case with noaddition of an acid or a coagulant, and the case with addition of only acoagulant.

Example 4

An excess sludge of a sewage disposal plant was concentrated by thecentrifugal concentrating method. The excess sludge had an SSconcentration of 0.8% (in average) and a PO₄—P concentration of 34 mg/l.Sulfuric acid was added as an acid so that the separated liquid had a pHof 5. Poly ferric sulfate was added as a coagulant in a fixed amount of15% relative to the solid content. As a Comparative Example,concentration was carried out by adding only a coagulant (poly ferricsulfate) in a fixed amount of 20% without adding an acid or a coagulant.The result is shown in Table 4.

TABLE 4 PO₄—P Solids concentration Concentration Separated of of liquidseparated Concentrated concentration liquid sludge Acid or coagulant not2160 mg/l 34 mg/l 1.8% added (Comparative Example) Acid not added and1100 mg/l  5 mg/l 3.1% coagulant added (Comparative Example) Acid andcoagulant  190 mg/l  3 mg/l 4.4% added (Example of Invention)

As is evident from Table 4, in the Example of the present invention inwhich an acid and a coagulant were added, the SS concentration ofsupernatant water was lower, with a better concentration efficiency, andPO₄—P could sufficiently be removed as compared with the case with noaddition of an acid or a coagulant, and the case with addition of only acoagulant.

Example 5

A mixed sludge resulting from mixture of sludge in a primary settlingtank and an excess sludge of a sewage disposal plant was concentrated byusing the above-mentioned filter cloth filtering concentrator. The mixedsludge had an SS concentration of 0.3% (in average) and a PO₄—Pconcentration of 41 mg/l. Sulfuric acid was added as an acid so that thefiltrate had a pH of 5.5. Poly ferric sulfate was added as a coagulantin a fixed amount of 15% relative to the solid content. As a ComparativeExample, only a coagulant (poly ferric sulfate) was added in a fixedamount of 20% relative to the solid content. Sludge was concentrated asanother Comparative Example without adding an acid or a coagulant. Theresult is shown in Table 5.

TABLE 5 Solids Concentration PO₄—P of Filtrate concentrationConcentrated concentration of filtrate sludge Acid or coagulant not 1160mg/l  41 mg/l 2.8% added (Comparative Example) Acid not added and  190mg/l 0.5 mg/l 4.0% coagulant added (Comparative Example) Acid andcoagulant  40 mg/l 0.1 mg/l 4.1% added (Example of Invention)

As is clear from Table 5, in Example of the present invention in whichan acid and a coagulant were added, the SS concentration of separatedliquid was lower, with a better concentration efficiency, and PO₄—Pcould sufficiently be removed as compared with the case with no additionof an acid or a coagulant, and the case with addition of only acoagulant.

Another sludge concentration apparatus of the present invention will nowbe described with reference to the drawings.

FIG. 6 is a perspective view, as viewed from the filter cloth descendingside, illustrating still another sludge concentrating apparatus of thepresent invention, wherein sludge uniform feeding means is provided;FIG. 7 is a sectional view illustrating further another sludgeconcentrating apparatus of the present invention, wherein sludge uniformfeeding means is provided; FIG. 8 is a perspective view, as viewed fromthe filter cloth descending side, illustrating another sludgeconcentrating apparatus of the present invention, wherein a filtratechamber is provided in the sludge tank; FIG. 9 is a perspective view, asviewed from the filter cloth ascending side, illustrating another sludgeconcentrating apparatus of the present invention, wherein a sludgeuniform discharge means is provided; FIG. 10 is a sectional viewillustrating another sludge concentrating apparatus of the presentinvention, wherein sludge uniform discharge means is provided; FIG. 11is a sectional view illustrating a sludge concentrating apparatus of thepresent invention having cleaning spray nozzles arranged in a staggeredform; FIG. 12 is a plan view illustrating cleaning means having cleaningspray nozzles arranged in a staggered form; FIG. 13 is a perspectiveview illustrating cleaning means having cleaning spray nozzles arrangedin a staggered form; FIG. 14 is a plan view illustrating cleaning meanshaving oscillatory cleaning spray nozzles; and FIG. 15 is a perspectiveview illustrating cleaning means having oscillatory cleaningspray-nozzles.

In FIGS. 6 to 11, the same reference numerals as in FIGS. 1 to 4represent the same component parts, and a detailed description thereofis omitted here, More specifically, reference numeral 8 represents asludge tank; 9, a sludge feed pipe; 10, a concentrated sludge dischargepipe; 11, a filtrate chamber; 11A, an exit side chamber; 12, a filtratedischarge pipe; 12A, a riser tube; 13, a weir; 14, a filtrateintroducing port; 15, a filtering means; 16, a lower roller; 17, anupper roller; 18, a filter cloth; 19, a cleaning means; 20, a tensioningroller; 21, filter cloth tensioning means; 22, sealing means; 23, asludge distributing tank; 24, solid stripping means; 25, a concentratedsludge discharge tank; 26, cleaning discharge water collecting means;27, driving means; and 28, an inlet side weir, respectively.

The sludge concentrating apparatus of the present invention shown inFIGS. 6 to 11 is characterized in that sludge uniform feeding means isprovided in the sludge distributing tank 23. The sludge uniform feedingmeans comprises a combination of a flow adjustment device 39 with theinlet side weir 28 provided at the sludge flow exit of the sludgedistributing tank 23, but the flow adjustment device 39 alone isapplicable. The inlet side weir 28 is a submerged weir so as to maintainthe cohesive state of sludge. The flow adjustment device 39 is formedinto a cylindrical shape so as to ensure smooth supply of sludge. Thesludge fed from the sludge feed pipe 9 by the sludge uniform feedingmeans is supplied uniformly in the width direction of the filter cloth18 in the sludge tank 8.

Another feature of the sludge concentrating apparatus of the presentinvention lies in the cleaning means 19. The cleaning means 19 must havea structure capable of permitting cleaning of the surface and back ofthe filter cloth 18 and preventing uneven cleaning. In the cleaningmeans 19 having such a structure, for example as shown in FIGS. 12 and13, an inside header pipe 19B having a plurality of inside spray nozzles19A attached thereto at certain intervals is horizontally (in the filtercloth width direction) arranged in the inside of the filter cloth 18,and two outside header pipes 19D, each having a plurality of outsidespray nozzles 19C attached at certain intervals are horizontally (in thefilter cloth width direction) arranged to the outside of the filtercloth 18, and outside spray nozzles 19C of the neighboring outsideheader pipe 19D are arranged alternately in a staggered form. Aplurality of inside header pipes 19B may be arranged with inside spraynozzles 19A arranged in a staggered form. Or, a plurality of headerpipes may be arranged in and outside, with inside and outside spraynozzles arranged in a staggered form. Further, a plurality of spraynozzles 19A may be arranged in a staggered form on a single header pipe19B.

As shown in FIGS. 14 and 15, the outside header pipe 19D may beoscillated horizontally (in a filter cloth width direction). In thiscase, the inside header pipe 19B may be oscillated, or both the insideand outside header pipes 19B and 19D may be oscillated.

Still another feature of the sludge concentrating apparatus of thepresent invention lies in that the sludge uniform discharge means isprovided in the concentrated sludge discharge tank 25. As shown in FIGS.9 and 10, the sludge uniform discharge means comprises a roller 40 and ascraper 41 serving as deposit stripping means which strips off depositadhering to the roller 40. The sludge uniform discharge means forcedlydischarge deposit simultaneously with the concentrated waste liquid sothat deposit mixed in the concentrated waste liquid discharged from theexit side weir 31 is prevented from adhering to the exit side weir 31and from being an obstacle to discharge of the concentrated wasteliquid. Since the concentrated waste liquid having a high concentrationis low in fluidity, it is necessary to discharge the same from thesludge tank 8 uniformly in the width direction of the sludge tank 8. Theroller 40 of the sludge uniform discharge means is therefore installedhorizontally in parallel with the exit side weir 31. The roller 40 isrotated by driving means 42 or in conjunction with driving means 27.

Further another feature of the sludge concentrating apparatus of thepresent invention lies in that, as shown in FIG. 7, the distance betweenthe inner wall surface of the sludge tank 8 at the concentrated sludgedischarge side and the outer surface of the filter cloth 18 is smallerthan the distance between the inner wall surface of the sludge tank 8 atthe sludge feeding side and the filter cloth 18 outer surface. Morespecifically, the former is ⅓ to ⅕ the latter. This permits stabledischarge even when disposing of a high concentration sludge.

The configuration having the above-mentioned features is of courseapplicable to the sludge concentrating apparatus shown in FIGS. 1 to 4.

According to the present invention, as described above, the followingadvantages are provided:

(1) Since solid/liquid separation and concentration of sludge of asewage disposal plant can be continuously carried out in a large amount,the sludge concentrating apparatus of the present invention isapplicable in place of a conventional mechanical concentrating equipmentsuch as a gravity thickening tank or a centrifugal concentrator;

(2) The sludge concentrating apparatus of the present invention has alarge disposal ability per area of installation and requires only a lowpower consumption. Therefore, when applying it in place of aconventional gravity thickener, it is possible to largely reduce thearea of installation so far required, and when applying it in place of aconventional centrifugal concentrator, the disposal cost can be largelycurtailed.

(3) The solid and phosphorus concentrations of filtrate can be largelyreduced as compared with the case of the conventional mechanicalconcentrator.

(4) A stable concentration of concentrated sludge is available, notdepending upon properties of sludge. It is therefore possible to safelyoperate the dehydrator during the dehydrating step of the latter stageof the process, and to downsize the dehydrating equipment.

What is claimed is:
 1. A sludge concentrating apparatus comprising: asludge tank which stores sludge; filtering means comprising a filtercloth that is shaped as an endless belt, and a plurality of rollerswhich turn the cloth, at least a lowermost one of the rollers beingimmersed in said sludge; a filtrate chamber including a filtrateintroducing port through which filtrate from the filtering means flowsinto the filtrate chamber, the filter cloth being kept in close contactwith the filtrate introducing port; cleaning means for removing sludgecakes adhering to the filter cloth; means for keeping a level of liquidin the filtrate chamber substantially constant; a sludge introducingmeans for introducing sludge to the sludge tank, said sludge introducingmeans being provided near an upper end of the filtrate introducing port;and concentrated sludge discharge means for discharging sludge from thesludge tank, said concentrated sludge discharge means being provided ata position substantially opposite to the sludge introducing means withthe filter cloth therebetween; wherein the means for keeping the liquidlevel in the filtrate chamber substantially constant comprises one of:(a) a weir provided at a boundary between the filtrate chamber and anexit chamber into which the filtrate from the filtrate chamber flows,and (b) a riser tube which extends vertically in the filtrate chamberand communicates with a discharge pipe through which filtrate isdischarged from the filtrate chamber.
 2. The sludge concentratingapparatus according to claim 1, further comprising: coagulant addingmeans for adding coagulant to the sludge fed into the sludge tank; andacid adding means for adding acid to the sludge fed into the sludgetank.
 3. The sludge concentrating apparatus according to claim 2,wherein the acid is sulfuric acid, and the coagulant is poly ferricsulfate.
 4. The sludge concentrating apparatus according to claim 3,wherein the sludge introducing means comprises sludge uniform feedingmeans for causing the sludge fed into the sludge tank to be feduniformly in a width direction of the filter cloth.
 5. The sludgeconcentrating apparatus according to claim 2, wherein the sludgeintroducing means comprises sludge uniform feeding means for causing thesludge fed into the sludge tank to be fed uniformly in a width directionof the filter cloth.
 6. The sludge concentrating apparatus according toclaim 1, wherein the sludge introducing means comprises sludge uniformfeeding means for causing the sludge fed into the sludge tank to be feduniformly in a width direction of the filter cloth.
 7. The sludgeconcentrating apparatus according to claim 6, wherein the sludge uniformfeeding means comprises at least one of a submerged weir and a flowadjustment device.
 8. The sludge concentrating apparatus according toclaim 1, wherein a distance between an inner wall surface of the sludgetank at a concentrated sludge discharge side thereof and an outersurface of the filter cloth at the concentrated sludge discharge side issmaller than a distance between an inner wall surface of the sludge tankat a sludge introducing side thereof and an outer surface of the filtercloth at the sludge introducing side.
 9. The sludge concentratingapparatus according to claim 1, wherein the concentrated sludgedischarge means comprises sludge uniform discharge means for causing thesludge discharged from the sludge tank to be discharged uniformly in awidth direction of said sludge tank.
 10. The sludge concentratingapparatus according to claim 9, wherein the sludge uniform dischargemeans is provided downstream of an exit weir of the concentrated sludgedischarge means and comprises a roller and deposit stripping means forstripping off deposit adhering to the roller.
 11. The sludgeconcentrating apparatus according to claim 1, wherein the cleaning meanscomprises a plurality of cleaning spray nozzles arranged in a staggeredform, which are fed from at least one header pipe.
 12. The sludgeconcentrating apparatus according to claim 1, wherein the cleaning meansis adapted to oscillate.